The present invention relates to a hydraulic directional valve for controlling the motion of a hydraulic cylinder or similar device, and more particularly to such a hydraulic directional valve having two valve spools. Hydraulic directional valves are well known in general. Typically, such valves have a single valve spool that is precisely machined to a close tolerance with the surrounding valve body. Close tolerances are generally required to prevent the leakage of pressurized hydraulic fluid past the valve spool when the spool is shifted in one direction or the other. Any leakage prevents maximum hydraulic pressure from reaching a device connected to the valve, and additionally, if the valve is used to hold a device in position, leakage may allow the device to creep.
Hydraulic directional valves are typically operated by manually moving an actuator, such as a lever, or may also be operated by means of an electronic solenoid. In a typical single-spool valve, operation is accomplished by shifting the valve spool in one direction or the other to allow pressurized hydraulic fluid to travel through a certain port or ports in the valve body, while blocking access to other ports. Many of these valves may also possess a center, or neutral position, where pressurized hydraulic fluid is allowed to enter the valve body and then routed directly back to a hydraulic tank that is part of the hydraulic system to which the valve is connected. Alternatively, such valves may also have a center position wherein certain ports are blocked. In this case, when the valve spool is moved to the center position, any hydraulic fluid that has passed through the valve body to the device connected to the valve is trapped between the device and the valve. This allows pressure to remain in the line connecting the device to the valve. In this latter embodiment, it is especially important that leakage between the valve spool and valve body is minimized. Any such leakage will allow the hydraulic pressure between the valve and the device to diminish, leading to movement of the device or a loss of force exerted thereby.
The requirement of close tolerances is problematic in several respects. First, close tolerance machining is costly and results in a significant increase in the price of a valve manufactured in such a manner. Second, such valves are difficult to repair properly because the original valve spools are matched to the valve body in which they are installed. The likelihood of a replacement valve spool fitting an arbitrary valve body is low. Additionally, these valves are typically very sensitive to contamination. Because of the close tolerances required, even small amounts of contamination can effect shifting of the valve spool or contribute to leakage between the valve spool and the valve body.
Therefore, a need exists for a hydraulic directional valve that is less costly to manufacture, that may be more easily repaired, and that is more resistant to contamination than current hydraulic directional valves. The present invention discloses such a valve. The dual-spool hydraulic directional valve of the present invention is particularly suited to applications where the valve is not required to hold a load. The valve of the present invention uses two valve spools, with each valve spool controlling flow of hydraulic fluid through a particular port or ports. The valve spools work independently from one another, therefore, the slight leakage of hydraulic fluid from one valve spool to the other will not markedly affect the operation of the valve. By utilizing dual valve spools, the valve of the present invention may be manufactured without the need for the close tolerances typically required between the valve spool and the valve body. The valve may also be more easily and successfully repaired, and is less likely to effected by contamination.
In a preferred embodiment of the valve, a lever is utilized to shift the positions of the respective valve spools, although other means, such as electronic solenoids may also be used. Movement of the lever in one direction will allow the flow of pressurized hydraulic fluid through a predetermined port or ports associated with the active spool, while blocking the flow of hydraulic fluid through the port or ports associated with the inactive spool. Movement of the lever in the opposite direction will reverse the role of the respective spools. Preferably, the valve of the present invention will also have a center, or neutral position, wherein hydraulic fluid may flow through the valve body and back to a hydraulic tank without passing to any device connected to the valve.